The QEU files are a 3D map of the deviation of the detector QE
from the nominal 1D vector of QE vs energy. This 1D vector refers to
the 0th order aim point. The three dimensions of the QEU file are
then detector x,y and energy. The CIAO software obtains the QE at any
detector location and energy by multiplying the nominal QE by the
value in the QEU map at that detector location and energy.

QE Decline:

The accumulation of LETG+HRC-S observations of the hot white dwarf
HZ43 since launch has revealed a slow decline in the HRC-S QE at a
rate of approximately half a percent per year. The decline appears to
be a bit faster in the 0th order (aim point) region of the detector
than for the regions probed by dispersed photon events. Since the
nominal detector QE is a 1D vector, a spatially non-uniform QE
decline can only be accounted for in the QEU map.

Because the decline is relatively slow - half a percent or so per
year - the decline can be adequately represented by annual QEU
files. Each QEU file represents the state of the QE decline over the
detector in the year in question. For an observation obtained in a
given year, CIAO will then employ the QEU map appropriate for that
year. Files are indexed to mid-year, so that the error in QE incurred
by the yearly digitization amounts to no more than 0.25% or so.

PIPELINES/TOOLS AFFECTED:

THREADS AFFECTED:

III. TECHNICAL DETAILS

A. HRC-S Time-displaced QEU maps version N0006

Calibration of the QE Decline:

In order to produce the new QEU files, we analysed all the
LETG+HRC-S observations of HZ43, supplemented with observations of
Sirius B. Both of these stars should be completely stable in X-ray
output, and so provide standard candles for LETGS monitoring.

The count rates obtained in dispersed and 0th order regions were
obtained for each observation using standard CIAO processing. Secular
decline in QE appears to be linear to within the accuracy of the
data. Linear fits to the 0th order and dispersed data provided the
rate of QE decline in each region.

QEU maps were produced to reflect the observed rate of QE decline
in the mid-years since the beginning of Chandra observations. Files
were placed into the test CALDB, and all observations we re-analysed
using CIAO to compute effective areas and derive fluxes for dispersed
and 0th order regions. These fluxes were verified to remove the
secular QE trend and produce a flat flux vs time.

The bounds of the 0th order region have been reset to S2 chipx =
[1776.5, 2416.5], chipy = [8544.5,9312.5]. The results of the
analysis are summarised in Figure 1 that shows the HZ43 observations
scaled by the effective areas in the current CIAO release, and using
the new QEU files in the test CALDB. The secular trend in the scaled
count rates is removed for both 0th order and dispersed events.

Fig. 1: Relative fluxes calculated from HX43 observations,
normalized to the July 2008 observation. The values in the upper plot
are for the zeroth order. The values in the lower plot are for the
dispersed spectrum.

Matching CIAO and CALDB Versions

News

CalDB 4.6.7
an upgrade to CalDB 4.6.5 and later versions, has been released. Notable additions include
the ACIS T_GAIN Epoch 60 quarterly upgrades (Nov-Jan 2015), and a correction to the HRC index
file. See the CalDB 4.6.7
Release Notes for details.

16 Dec 2014

CalDB 4.6.5,
an upgrade to CalDB version 4.5.9 and later versions, has been released. Notable additions
included are ACIS T_GAIN Epoch 59 updates (Aug-Oct 2014), HRC-I GMAP and PIBGSPEC updates for 2014,
and HRC-S QE version N0013 and QEU (version N0007) modifications. See the
CalDB 4.6.5 Release
notes for details.

CalDB 4.6.3,
an upgrade to CalDB version 4.5.9 and later versions, has been released. Notable additions
included are ACIS T_GAIN Epoch 58 updates (May-July 2014), HRC-S QEU files for 2014-2020,
mitigation of the HRC-S QEU Caveats.
CalDB 4.6.3 Release
notes for details.

25 Jul 2014

HRC-S QEU File Search Error Caveat
added. This applies to all HRC-S observations taken in 1999 and in 2014. A workaround is
prescribed on this page. The fix will be released in mid August 2014. NOTE: These have been
mitigated with the CalDB 4.6.3 release on 29 Aug 2014!

09 Jul 2014

CalDB 4.6.2, an upgrade to CalDB version 4.5.9
and later versions, has been released. It contains (a) an ACIS quarterly T_GAIN
upgrade (Epoch 57, Feb-Apr 2014), (b) upgrades to PCAD CALALIGN files for 2003, 2006, 2012,
and 2013, and finally (c) an upgrade to the ACIS CONTAM file to improve on-axis calibration
performance from April 2013 onward. See the
CalDB 4.6.2 Release
notes for details.

09 Apr 2014

CalDB 4.6.1.1, a patch upgrade to CalDB 4.6.1
which corrects the HRC branch index listings of the latest two HRC-I background event lists
for 2012 and 2013, so that they are selected appropriately for OBS_IDs taken after
(UTC) 2012-01-01T00:00:00. See the
CalDB 4.6.1.1 Release
notes for details.

26 Mar 2014

CalDB 4.6.1, an upgrade to CalDB version 4.5.9
and later versions, has been released. It contains (a) an ACIS quarterly T_GAIN
upgrade (Epoch 56, Nov-Jan 2014), as well as (b) a new HRC-I BKGRND event list library
which adds new files for the four most recent calendar years (2010-2013). See the
CalDB 4.6.1 Release
notes for details.